Even though there has been considerable study of alternative electrochemical systems, the lead-acid battery is still the battery of choice for general purposes, such as starting a vehicle, boat or airplane engine, emergency lighting, electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware, both industrial and military. These batteries may be periodically charged from a generator.
The conventional lead-acid battery is a multi-cell structure. Each cell generally comprises a set of vertical positive and negative plates formed of lead or lead alloy grids containing layers of electrochemically active pastes. The paste on the positive plate when charged comprises lead dioxide, which is the positive active material, and the negative plate contains a negative active material such as sponge lead. An acid electrolyte, based on sulfuric acid, is interposed between the positive and negative plates.
Lead-acid batteries are inherently heavy due to use of the heavy metal lead in constructing the plates. Modern attempts to produce light-weight lead-acid batteries, especially in the aircraft, electric car and vehicle fields, have placed their emphasis on producing thinner plates from lighter weight materials used in place of and in combination with lead. The thinner plates allow the use of more plates for a given volume, thus increasing the power density.
Higher voltages are provided in a bipolar battery including bipolar plates capable of through-plate conduction to serially connected electrodes or cells. The bipolar plates must be impervious to electrolyte and be electrically conductive to provide a serial connection between electrodes.
U.S. Pat. Nos. 4,708,918 and 4,861,689 discuss various aspects of bipolar plates and batteries.
One problem which has presented itself in considering bipolar plate batteries or bipolar batteries is the addition of electrolyte to the assembled battery. Such a bipolar battery includes a plurality of bipolar cells, each of which include a negative electrode, a positive electrode, a separator between the electrodes to separate the electrode and hold electrolyte, and a fluid impervious, electrically conductive bipolar plate adjacent each electrode. Thus, each bipolar plate may be considered to be a part of two bipolar cells. The thickness of such bipolar cells is often significantly less than the thickness of conventional monopolar cells, such as in the range of about 0.080 inches to about 0.300 inches per cell. Such reduced thickness makes filling each of the bipolar cells, which are to be isolated against fluid flow one from the other, with a controlled amount of electrolyte quite difficult, particularly at fill rates to satisfy commercial production schedules and/or using commercially available equipment. A second problem presented is to effectively limit the liquid and/or gaseous components from one bipolar cell from interfering with the functioning of another bipolar cell. Such cell to cell interference can result in a reduction in the overall efficiency of the bipolar battery, or even in battery failure.
Another problem with bipolar batteries is that of maintaining the spacing between positive and negative electrodes. Maintaining the interelectrode spacing provides substantial performance advantages. For example, substantially uniform spacing between the positive and negative electrodes provides for a substantially uniform current distribution and flow between the bipolar cells and electrode pairs which are included in the assembled bipolar battery. Such spacing is particularly important in sealed maintenance free oxygen recombinant batteries. However, as the dimensions of the bipolar plate surfaces associated with the positive and negative electrodes increase, particularly the surface area dimension, the more difficult it becomes to maintain proper interelectrode spacing. It is often useful to make such surfaces relatively large in order to provide a bipolar battery with the desired capacity.
The present invention addresses these and other problems and concerns as will become apparent hereinafter.